Using a transformer to change 220V to 12V, can the 12V lithium battery be charged after rectification?

by:CTECHi     2021-07-30

Absolutely not!

Because of its high energy density and special chemical properties, lithium batteries must use a power management chip for safety and life considerations, that is, a charging chip (chargerIC) .

In addition, the charging chip can optimize the charging curve to the greatest extent. Together with the software algorithm, it can ensure the battery life under fast charging conditions and greatly improve the charging process. safety. Let's first look at a commonly used charging chip.

Power Management

This is a charging IC of TI (Texas Instruments), which is mainly used for consumer electronic products When charging the lithium battery, the red part is the lithium battery pack (some have a temperature sensor inside).

The chip supplies power to the battery pack through the OUT pin, and is also the main power supply of the entire system. Its ISET pin can set the maximum charging current to prevent the current from exceeding the battery. The maximum value the bag can withstand. Let's continue to look at the internal structure of the charging IC.

Charging chip

This makes it clearer that the part marked with red plays a key role in the charging process of lithium batteries. OUT terminal, it can detect the voltage value of the battery pack in real time, plus the calculation of the charging current, you can get the current power of the battery pack (equivalent to a fuel gauge). When the current state of the battery pack is determined, the revised fuel gauge model can be used to determine which charging mode to use.

Charging curve

What is the charging mode? Here is a brief introduction to science, our friends who like to play digital should often hear about mobile phone fast charging (what is the terminology such as charging for five minutes, talking for two hours...).

Fast charge generally refers to the 3C charging mode. For example, the capacity of a lithium battery is 580mAh, when the charging current is 3 times the battery capacity value (3*580 ), that is, 3C charging, about 1.7A charging current. Understand clearly what is fast charging, the next step is the common lithium battery charging mode.

This is a charging curve, looking from left to right (the dashed line is the voltage, the solid line is the current), when the battery is first charged, the charging current is very small, which is equivalent to 'Preheating' the battery can extend battery life. This stage is called 'trickle charge

When the battery is ready, you can start high-current (3C) charging. This stage is called 'constant current chargingIf the temperature of the bag is too high, the charging current will drop to 2C or 1.5C, etc. When the temperature drops, 3C charging is resumed, and so on.

If the battery pack voltage reaches a certain inflection point (usually determined by the chip), such as a 12V battery, assuming its inflection point is 9V, that is, when the battery pack voltage When it reaches about 9V, the charging chip will enter the next stage, that is, 'constant voltage charging'. At this time, the charging voltage value changes very little, but the current value drops quickly.

This principle is very simple. For example, we use a bucket to fill water. At first, don’t open the faucet too much (trickle), because the water in the empty bucket will splash. When there is a certain water level at the bottom of the bucket, you can turn on the faucet to the maximum (constant flow) and inject water at the fastest speed.

Finally, the water in the bucket is almost full. In order to prevent the water from spilling out, should we turn down the tap? Water flow is like electricity, which is 'constant voltage charging'.

Battery model

The 'cut-off' of the last step of charging is also very important. Each charging chip has a default cut-off Voltage (some are adjusted by software). How do we judge when the charging should be cut off? This requires a new concept of popular science: battery model.

Because the charge calculated by the charging chip or fuel gauge is different from the actual charge of the battery. The power value we calculated is only an ideal value, and we need to combine the battery model provided by the battery manufacturer to comprehensively judge the power value of the battery.

If the deviation between the calculated value and the actual value is too large, you need to modify the fuel gauge model according to the battery model to bring the calculated value close to the actual value infinitely.

The battery model given by the manufacturer is based on complex verification tests, such as battery high and low temperature tests, high altitude tests, and tests such as instantaneous temperature changes. It is used to verify the capacity changes of the battery in different environments, and finally this accurate battery model is obtained.

According to this model, we can estimate the current capacity of the battery and when it will be fully charged, and the 'cut-off' point can be calculated.

So back to your question above, even if you get a stable DC 12V power supply through perfect BUCK, but there is no power management chip, no strict software algorithm, so what? In what state can the battery be guaranteed to be charged in what mode?

Furthermore, you can't even be sure whether the battery is fully charged (100%), because the final 1% charge often takes a long time.

In addition, from the perspective of charging safety, the inability to monitor the temperature inside the battery pack in real time will undoubtedly increase the safety hazard!

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